Outside vapor deposition apparatus for making optical fiber preform and method for making optical preform using the same

ABSTRACT

An Outside Vapor Deposition (OVD) apparatus for making an optical fiber perform with uniform deposition of silica particles through uniform heating to the overall length of the preform includes a mandrel having a predetermined length and driven to rotate and a burner for emitting a combustion gas toward the mandrel and burning the combustion gas to make silica particles to that the silica particles are deposited on a surface of the mandrel, wherein the burner has a length corresponding to the length of the mandrel and provides uniform temperature throughout the overall length of the mandrel at the same time.

TECHNICAL FIELD

[0001] The present invention relates to an Outside Vapor Deposition(OVD) apparatus for making an optical fiber preform and a method formaking an optical fiber preform using the apparatus, and moreparticularly to an OVD apparatus which enables uniform deposition ofmaterial particles constituting an optical fiber preform over the lengthof the preform by using a burner having an improved structure and amethod for making an optical fiber preform using the apparatus.

BACKGROUND ART

[0002] An Outside Vapor Deposition (OVD) device is broadly used formaking an optical fiber preform since it may give a bigger-diameterpreform with high deposition efficiency.

[0003] An example of the conventional OVD device is schematically shownin FIG. 1. Referring to FIG. 1, the conventional OVD device includes acylindrical burner 12 mounted upon a plate 10, and a mandrel 18 mountedabove the burner 12 to rotate in a predetermined direction. While theOVD process is conducted, material particles constituting an opticalfiber preform 16 are deposited on the mandrel 18. In the OVD process,the cylindrical burner 12 supplied with combustion gas and reaction gasemits flame 14 toward the mandrel 18 in order to cause a hightemperature state thereto, and it is also reciprocated in the horizontaldirection. This process causes generation of fine particles of thematerial constituting the optical fiber preform, and the generatedparticles are deposited on the surface of the mandrel 18 in apredetermined thickness.

[0004] More specifically, combustion gases such as H₂ and O₂ andreaction gases such as SiCl₄ and GeCl₄ are supplied to the cylindricalburner 12 at a predetermined flow rate. Then, combustion reaction of thecombustion gases causes a high temperature state, and material particlessuch as SiO₂ and GeO₂ are generated. The generated particles aredeposited on the surface of the rotating mandrel 18 in a predeterminedthickness.

[0005] The material particles such as SiO₂ and GeO₂ are generated whenthe reaction gases are hydrolyzed with a burning product H₂O or directlyoxidized at or above 1100° C. with a carrier gas O₂ formed by the burner12 according to the chemical reaction formula expressed below. The fineparticles of SiO₂ and GeO₂ collide into each other and condense intoparticles with a diameter of about 0.2μm, and are deposited on thesurface of the rotating mandrel 18.

[0006] Chemical Reaction Formula 1

[0007] The deposition mechanism of the fine material particlesconstituting the optical fiber preform in the optical fiber preformmanufacturing process using the OVD device is thermophoresis.Thermophoresis means that, when fine particles exist in a gas having atemperature gradient, the particles move from a high temperature area toa low temperature area due to the momentum exchange between particlesand gas molecules. The rate of the thermophoresis is calculatedaccording to the following Mathematical Equation 1.

V _(t)=−(Kν/T)/ΔT  Mathematical Equation 1

[0008] Here, K is a thermophoresis constant.

[0009] As shown in the above Mathematical Equation 1, it will be knownthat the temperature gradient is a main factor to the particledeposition in the optical fiber preform making process using the OVDdevice. In other words, the combustion of hydrogen and oxygen emittedfrom the burner 12 makes the reaction gas be oxidized and the reactiongas be hydrolyzed by the flame near the burner 12 to form fine materialparticles constituting the optical fiber preform 16, and these particlesmove together with hot gases emitted from the burner 12 and pass aroundthe mandrel 18. These particles are then deposited to the mandrel 18having a relatively low temperature due to the effect of temperaturegradient. Thus, the particle deposition efficiency is increased as theparticle has higher temperature and the mandrel 18 has lowertemperature.

[0010] In the OVD process, whenever moving on the plate 10, thecylindrical burner 12 changes compositions of SiCl₄ and GeCl₄ so thatthe optical fiber preform 16 may obtain a desired refractive index, inthe general OVD process. In addition, the mandrel 18 is separated andremoved from the preform 16 when the preform 16 has a desired depositionthickness. This preform 16 is then dried and sintered in a furnace whichis maintained at a temperature of 1400˜1600° C., so as to make anoptical fiber preform.

[0011] When executing the OVD process, one or multiple burners 12 arearranged in series, and then the burner(s) 12 or the mandrel 18 is movedlaterally. It is because the burner 12 used in the conventional OVDprocess has a cylindrical shape as shown in FIG. 2, and thus heats justa local area of the optical fiber preform 16, as shown in FIG. 2.

[0012] On the other hand, FIG. 4 shows planar distribution of the flame14 generated by the conventional cylindrical burner 12 equipped in theconventional OVD device. Referring to FIG. 4, it may be understood thatthe flame 14 is more focused in the center of the burner 12. In theplanar distribution of FIG. 4, darker area shows that the flame 14 ismore concentrated. Accordingly, the material particles deposited on themandrel 18 by the conventional cylindrical burner 12 have a densitygradient in a radial direction. This fact is also proven by FIG. 3 whichshows soot particle density in X- and Y-direction.

[0013] If the material particles of the optical fiber preform 16 aredeposited on the rotating mandrel 18 with laterally moving the burner12, a spiral deposition pattern 19 is created on the surface of themandrel 18, as shown in FIG. 5. This spiral deposition pattern 19 makesa deposition layer of a certain thickness whenever the burner 12 passes,and such deposition layers are stacked to form the optical fiber preform16. However, due to the above-mentioned spiral deposition pattern 19,portions having a high soot density are overlapped at a certainposition, and there are also generated non-overlapped portions on themandrel 18. Thus, the overlapped portion 19 a becomes relativelythicker, and the preform may hardly obtain a uniform thickness all overthe length. In addition, rapid transfer of the burner 12 or the preform16 may cause turbulence to a laminar flow of the flame, so there is alimit in improving the deposition efficiency. Moreover, ends of theoptical fiber preform 16 cannot be used because of irregularity of theparticle stream, thus causing losses.

[0014] The difference of the soot density may cause irregularity of thedeposited thickness, and such irregular deposition may cause overlaps.Such overlaps become a factor limiting a deposition speed, a depositionamount and a deposition density as the optical fiber preform 16increases, and eventually form ripples on an outer circumference of thefinished optical fiber preform 16 after the sintering process. Theripples formed on the surface of the optical fiber preform 16 causeinferiority in the frequency blocking characteristic and thedistribution characteristic which are sensitive to the core diameter, sothe ripples should be removed To solve such a problem, there is proposedU.S. Pat. No. 4,486,212 entitled “Devitrification Resistance FlameHydrolysis Process”. This document discloses a technique to guide silicaparticles to be uniformly deposited on the mandrel by decreasing aninitial deposition speed to a very low level. Although the patentedmethod may restrain the amplification of the irregular deposition tosome extent, but it cannot solve the above problem completely. Asdisclosed in Korean Patent Filing No. 92-19778, this technique cannotovercome the drawback that a loss at both ends of the optical fiberpreform reaches 20% due to reciprocation of the burner or preform, andthe control of deposition is difficult. In addition, the aboveconventional techniques cannot solve problems derived from the spiraldeposition pattern generated after the initial deposition, andinevitably leads to lower productivity caused by the initial depositioncontrol.

[0015] In addition, U.S. Pat. No. 4,683,994 entitled “Process andApparatus for Forming Optical Fiber Preform” suggests a technique usinga large-scale soot generator which covers all area of the optical fiberpreform. However, when using the method proposed in the above document,gases are mixed inside the soot generator, so problems such as sootgrowth and clogging of nozzles due to the grown soot arise. In addition,since the overall optical fiber preform gets increased temperature, theparticle deposition efficiency using the thermophoresis is evendecreased.

DISCLOSURE OF INVENTION

[0016] The present invention is designed to overcome the above problemsof the prior art, therefore an object of the invention is to provide anOutside Vapor Deposition (OVD) apparatus for making an optical fiberpreform, which employs a straight burner having a length equal orsimilar to an optical fiber preform to deposit material particles of theoptical fiber preform on the surface of the preform, thereby reducingdeposition time without a spiral deposition pattern and dramaticallyimproving the deposition efficiency. Another object of the presentinvention is to provide a method for making an optical fiber preformusing the apparatus .

[0017] In order to accomplish the above object, the present inventionprovides an Outside Vapor Deposition (OVD) apparatus for making anoptical fiber preform, which includes a mandrel having a predeterminedlength and is capable of rotation; and a burner for emittingpredetermined reaction gas together with combustion gas toward themandrel to generate material particles of an optical fiber preform sothat the material particles are deposited on a surface of the mandrel toform the optical fiber preform, wherein the burner has a lengthcorresponding to the mandrel and provides uniform temperature throughoutthe overall length of the mandrel at the same time.

[0018] Preferably, a flame generated by the burner is substantiallyfocused toward the central axis of the mandrel along all length of themandrel.

[0019] In addition, it is also preferred that the burner has a pluralityof chambers connected to different gas supply lines in order to emit thecombustion gas and the reaction gas independently. The chamber may becomposed of a gas spreading channel, and the gas spreading channelpreferably has a section which becomes substantially wider from a gasinput portion to a gas output portion.

[0020] A filter may be provided at the gas output portion of the gasspreading channel so as to disperse gas pressure in the gas spreadingchannel. In this case, the filter preferably has a pore size in therange of 50˜150μm.

[0021] In the OVD apparatus of the present invention, the gas supplyingline preferably supplies the combustion gas and the reaction gasindependently into the chamber.

[0022] In another aspect of the present invention, there is alsoprovided a method for making an optical fiber preform using an OutsideVapor Deposition (OVD) apparatus, which includes the steps of: preparinga mandrel having a predetermined length and a burner having a lengthcorresponding to the mandrel; rotating the mandrel at a c speed;emitting predetermined reaction gas and combustion gas toward themandrel through the burner so as to provide a flame having the sametemperature throughout overall length of the burner and cause generationof material particles of an optical fiber preform so that the materialparticles are deposited on the overall length of the mandrel at the sametime; and depositing the material particles of the optical fiber preformto a predetermined thickness, and then removing the mandrel to obtainthe optical fiber preform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and other features, aspects, and advantages of preferredembodiments of the present invention will be more fully described in thefollowing detailed description, taken accompanying drawings. In thedrawings:

[0024]FIG. 1 is a schematic front view showing an Outside VaporDeposition (OVD) device according to the prior art;

[0025]FIG. 2 shows a cylindrical burner and its flame used in the OVDdevice of FIG. 1;

[0026]FIG. 3 shows a density graph of soot particles formed by thecylindrical burner of FIG. 2;

[0027]FIG. 4 shows planar distribution of the flame emitted from thecylindrical burner of FIG. 2;

[0028]FIG. 5 shows a spiral deposition pattern generated on an opticalfiber preform by the cylindrical burner of FIG. 2;

[0029]FIG. 6 is a schematic front view showing an OVD apparatusaccording to the present invention;

[0030]FIG. 7 shows a burner and its flame used in the OVD apparatus ofthe present invention;

[0031]FIG. 8 shows a density graph of soot particles formed by theburner of FIG. 7;

[0032]FIG. 9 shows planar distribution of the flame emitted from theburner of FIG. 7;

[0033]FIG. 10 is a partially-sectioned front view showing the burner ofFIG. 7;

[0034]FIGS. 11a and 11 b are front and side views showing the flameemitted from the burner of FIG. 7, respectively, and

[0035]FIG. 12 shows that material particles of the optical fiber preformare deposited on the preform without any deposition pattern by using theburner of FIG. 7.

BEST MODES FOR CARRYING OUT THE INVENTION

[0036] Hereinafter, preferred embodiments of the present invention willbe described in detail with reference to the accompanying drawings.

[0037]FIG. 6 schematically shows an Outside Vapor Deposition (OVD)apparatus for making an optical fiber preform according to the presentinvention Referring to FIG. 6, the OVD apparatus of the presentinvention includes a mandrel 26 installed at a predetermined height, anda burner 20 installed below the mandrel 26. The mandrel 26 has a shapeof a long rod and is connected to a driving means (not shown) forrotation.

[0038] The burner 20 has a length corresponding to the mandrel 26,preferably equal to or longer than the length of the mandrel 26.

[0039] Gas supply lines 30 (see FIG. 10) are installed in the burner 20so as to supply reaction gases such as SiCl₄ and GeCl₄ for generation ofmaterial particles of an optical fiber preform 24 together withcombustion gases such as H₂ and O₂. In addition, the burner 20 generatesmaterial particles of the optical fiber preform 24 such as SiO₂ and GeO₂by means of hydrolysis or oxidization, and these material particles aredeposited on the surface of the rotating mandrel 26.

[0040] When the material particles of the optical fiber preform 24 aredeposited on the mandrel 26 by using the burner 20, the mandrel 26 keepsrotating at a high speed so that the material particles of the opticalfiber preform 24 may be uniformly deposited on the overall surface ofthe mandrel 26. Particularly, since the burner 20 of the presentinvention has a length corresponding to the mandrel 26, the materialparticles of the optical fiber preform 24 are uniformly deposited overthe whole length of the mandrel 26.

[0041]FIG. 7 shows the burner 20 used in the present invention and itsflame 22. Referring to FIG. 7, it will be easily known that the flame 22is also uniformly formed in a straight line owing to the shape of theburner 20. Thus, a deposition density of the material particles of theoptical fiber preform 24 caused by the burner 20 is distributed regularin an X axis over the whole length of a preform 24 and graduallydecreases in a Y axis from the center to an outside, as shown in FIG. 8.In addition, since the mandrel 26 keeps rotating during the depositionprocess of the material particles of the optical fiber preform 24, thedeposition density in the Y axis also becomes substantially regular onthe whole circumference of the optical fiber preform 24.

[0042]FIG. 9 shows a planar distribution of the flame 22 formed from theabove-mentioned straight burner 20 of the present invention. In FIG. 9,darker area shows that the flame 22 is more concentrated at thatportion. As would be easily known in the figure, the flame 22 givesuniform temperature along the center in a longitudinal direction of thestraight burner 20.

[0043]FIG. 10 is a partially sectioned view of the straight burner 20used in the present invention. Referring to FIG. 10, the straight burner20 of the present invention is composed of many chambers, each of whichis equipped with the gas supply line 30. The gas supply line 30 is usedfor supplying reaction gases such as SiCl₄ and GeCl₄ together withcombustion gases such as H₂ and O₂. The reaction gases and thecombustion gases may be supplied through one line at the same time, orthe gas supply lines 30 may also be provided for the reaction gases andthe combustion gases separately so that the reaction gases and thecombustion gases are not mixed before chemical reaction thereof.

[0044] The gas supply lines 30 are installed with a regular space inbetween, and each gas supply line 30 is connected to a gas spreadingchannel 32. The gas spreading channel 32 forms each chamber in theburner 20 and its section becomes wider from a gas input portion to agas output portion. Particularly, the gas output portions of the gasspreading channels 32 are preferably formed to cover the most part of anupper surface of the burner 20. In such a configuration, gases arescattered through the gas spreading channel 32 and thus lessenrelatively high pressure in the gas supply line 30. In addition, thegases scattered as above are then uniformly discharged to all area abovethe burner 20, thereby resultantly forming a uniform temperature fieldalong the longitudinal direction of the optical fiber preform 24. Thisenables uniform deposition of the material particles of the opticalfiber preform 24.

[0045] A filter 34 may be installed as well to the gas output portion ofthe gas spreading channel 32. This filter 34 is preferably a glassfilter having a pore size in the range of 50˜150μm, and this glassfilter 34 plays a role of dispersing gas pressure in the gas spreadingchannel 32.

[0046]FIGS. 11a and 11 b are front and side views showing the flame 22emitted from the straight burner 20 of the present invention. Referringto the figures, it would be known that the flame 22 generated by theburner 20 is approximately uniform over the length of the optical fiberpreform 24 and is focused on the center of the optical fiber preform 24when being seen from the side. In FIG. 11b, reference numeral 36 denotesan independent flow channel, which makes input gases be supplied abovethe burner 20 through multiple paths. Thus, the flame 22 generated bythe burner 20 is emitted through approximately overall area of the uppersurface of the burner 20, and the top of the flame 22 is substantiallyoriented to a lower center of the optical fiber preform 24.

[0047]FIG. 12 shows that the material particles of the optical fiberpreform 24 are uniformly deposited on the optical fiber preform 24without any deposition pattern by using the OVD apparatus of the presentinvention. In the OVD apparatus of the present invention, the burner 20or the optical fiber preform 24 does not move laterally. That is to say,the straight burner 20 makes the material particles be deposited on thewhole length of the preform 24 with just keeping rotation. Thus, in thepresent invention, the material particles are uniformly deposited on thesurface of the optical fiber preform 24 without a deposition pattern,particularly the conventional spiral pattern and after all, the opticalfiber preform 24 may obtain a uniform deposition density and a constantthickness over the whole length. In FIG. 12, reference numeral 25denotes the material particles, which are deposited on the optical fiberpreform 24.

[0048] Since the burner 20 of the present invention is designed to coverthe overall length of the optical fiber preform 24, there is no need ofreciprocation of the burner 20 and thus the disturbance of the flamelaminar flow can be prevented. In that reason, the material particlesare deposited to the whole length of the optical fiber preform 24, andthereby the optical fiber preform 24 can be deposited through all layerswithin a shorter time.

[0049] In addition, since the burner 20 is composed of multiplechambers, supplied amounts of the source gases can be controlled at thesame rate from the outermost part to the center of the burner 20.Furthermore, the fan-shaped gas spreading channels 32 dramatically lowerthe local pressure of the gas, thus the gas spreading channels 32 maybasically prevent the gas emitted from the gas supply lines 30 frombeing focused on a specific position.

[0050] Now, a method for making an optical fiber preform with the use ofthe OVD apparatus according to the present invention is described asfollows.

[0051] To make an optical fiber preform by the use of the OVD apparatusof the present invention, the mandrel 26 having a predetermined lengthis installed and the burner 20 having a length corresponding to themandrel 26 is also installed below the mandrel 26. In addition, themandrel 26 is controlled to rotate at a constant speed by using aseparate driving means.

[0052] Under such a working condition, the reaction gases such as SiCl₄and GeCl₄ are supplied toward the mandrel 26 through the gas supplylines 30 together with the combustion gases such as H₂ and O₂. At thistime, since multiple gas supply lines 30 are installed at a regularspace along a longitudinal direction of the burner 20, the reactiongases and the combustion gases are uniformly supplied through the wholelength of the burner 20. Furthermore, since the gas spreading channels32 and the filters 34 are formed at the end of the gas supply lines 30,local focusing of the reaction gases and the combustion gases arebasically prevented.

[0053] At this time, the burner 20 generates the flame 22 having thesame temperature through the whole length thereof so as to oxidize orhydrolyze the reaction gases, and thus there are generated materialparticles of the optical fiber preform 24 such as SiO₂ and GeO₂. Thesematerial particles are deposited on the surface of the mandrel 26,uniformly to the whole length of the mandrel 26. In addition, since themandrel 26 keeps rotating, the material particles are also depositeduniformly around the mandrel 26.

[0054] Such deposition of the material particles is continued until thedeposition layers formed around the mandrel 26 reach a desiredthickness. If obtaining a desired thickness of the silica particledeposition layer, the supply of the combustion gases and the reactiongases is stopped and the mandrel 26 quits rotation. After theseprocesses, the mandrel 26 is removed, and then the optical fiber preformaccording to the OVD process is complete. This optical fiber preform 24has a uniform thickness not only along the circumferential directionowing to the rotation of the mandrel 26 but also along the longitudinaldirection since the combustion gases and the reaction gases areuniformly supplied along the longitudinal direction.

Industrial Applicability

[0055] The OVD apparatus for making an optical fiber preform accordingto the present invention provides the flame of the same temperaturealong the overall length of the optical fiber preform at the same time.Thus, the OVD apparatus of the present invention may solve thesuccessive spiral deposition pattern problem of the optical fiberpreform caused by the deposition density difference and the unevendeposition surface problem caused by the deposition amount difference,which commonly appeare in the conventional optical fiber preformdeposition process.

[0056] In addition, the OVD apparatus of the present invention mayprevent generation of inferior optical characteristics sensitive to theoptical fiber core by restraining the outer surface unevenness of theoptical fiber preform.

[0057] Furthermore, since the burner has the length substantiallyidentical to the optical fiber preform, there is no need forreciprocation of the burner or the optical fiber preform in the OVDapparatus of the present invention. Thus, the OVD apparatus of thepresent invention may restrain fluctuation of the soot particles causedby the reciprocation of the burner or the optical fiber preform. Inaddition, it is also advantageous that the OVD apparatus may therebyremove a local difference of the soot density and reduce the loss at theends of the finished optical fiber preform.

[0058] In addition, since there is not required a time for reciprocationof the burner or the optical fiber preform, the OVD apparatus of thepresent invention may dramatically reduce the time required for thedeposition of the material particles of the optical fiber preform.

[0059] The present invention has been described in detail. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

What is claimed is:
 1. An Outside Vapor Deposition (OVD) apparatus formaking an optical fiber preform comprising: a mandrel having apredetermined length and capable of rotating; and a burner for emittingpredetermined reaction gas together with combustion gas toward themandrel to generate material particles of an optical fiber preform sothat the material particles are deposited on a surface of the mandrel toform the optical fiber preform, wherein the burner has a lengthcorresponding to the mandrel and provides uniform temperature throughoutthe overall length of the mandrel at the same time.
 2. An OVD apparatusaccording to claim 1, wherein a flame generated by the burner issubstantially focused toward the central axis of the mandrel along thelength of the mandrel.
 3. An OVD apparatus according to claim 1, whereinthe burner has a plurality of chambers connected to different gas supplylines in order to emit the combustion gas and the reaction gasindependently.
 4. An OVD apparatus according to claim 3, wherein thechamber is composed of a gas spreading channel.
 5. An OVD apparatusaccording to claim 4, wherein the gas spreading channel has a sectionwhich becomes substantially wider from a gas input portion to a gasoutput portion.
 6. An OVD apparatus according to claim 4 or 5, wherein afilter is provided at the gas output portion of the gas spreadingchannel so as to disperse gas pressure in the gas spreading channel. 7.An OVD apparatus according to claim 6, wherein the filter has a poresize in the range of 50˜150 μm.
 8. An OVD apparatus according to any ofclaims 3 to 7, wherein the gas supplying line supplies the combustiongas and the reaction gas independently into the chamber.
 9. An OVDapparatus according to any of claims 1, wherein the material particlesinclude silica particles or germanium oxide.
 10. A method for making anoptical fiber preform using an Outside Vapor Deposition (OVD) apparatuscomprising the steps of: preparing a mandrel having a predeterminedlength and a burner having a length corresponding to the mandrel;rotating the mandrel at a certain speed; emitting predetermined reactiongas and combustion gas toward the mandrel through the burner so as toprovide a flame having the same temperature throughout overall length ofthe burner and cause generation of material particles of an opticalfiber preform so that the material particles are deposited on theoverall length of the mandrel at the same time; and depositing thematerial particles of the optical fiber preform to a predeterminedthickness, and then removing the mandrel to obtain the optical fiberpreform.
 11. A method for making an optical fiber preform according toclaim 10, wherein a flame generated by the burner is substantiallyfocused toward a central axis of the mandrel along all length of themandrel.
 12. A method for making an optical fiber preform according toclaim 10, wherein the burner has a plurality of chambers connected todifferent gas supply lines in order to emit the combustion gas and thereaction gas independently.
 13. A method for making an optical fiberpreform according to claim 12, wherein the chamber is composed of a gasspreading channel.
 14. A method for making an optical fiber preformaccording to claim 13, wherein the gas spreading channel has a sectionwhich becomes substantially wider from a gas input portion to a gasoutput portion.
 15. A method for making an optical fiber preformaccording to claim 13 or 14, wherein a filter is provided at the gasoutput portion of the gas spreading channel so as to disperse gaspressure in the gas spreading channel.
 16. A method for making anoptical fiber preform according to claim 15, wherein the filter has apore size in the range of 50˜150 μm.
 17. A method for making an opticalfiber preform according to any of claims 12 to 16, wherein the gassupplying line supplies the combustion gas and the reaction gasindependently into the chamber.
 18. A method for making an optical fiberpreform according to claim 10, wherein the material particles includesilica particles or germanium oxide.